Photocatalysts for decomposition of water by visible light

ABSTRACT

A photocatalyst which contains at least rhombic tantalum nitride or consists of rhombic tantalum nitride. A photocatalyst wherein said photocatalyst loads a promoter composed of transition metal, in particular, the photocatalyst wherein a transition metal is platinum, further the photocatalyst for decomposition of water comprising any of these photocatalysts.

FIELD OF THE INVENTION

[0001] The present invention relates to a novel photocatalyst. Namely,this invention relates to a photocatalyst which contains at leastrhombic tantalum nitride, particularly, a visible-light activephotocatalyst consisting of rhombic tantalum nitride and a photocatalystfor the water-splitting.

BACKGROUND OF THE INVENTION

[0002] The following photocatalytic reaction is well-known as atechnique to obtain an aimed subject. That is, the light is irradiatedto a solid compound which has a photocatalytic function so as togenerate exited electrons and holes. Then a substance is oxidized orreduced by said exited electrons and holes and obtain the aimed object.

[0003] In particular, photocatalytic decomposition of water, isinterested in from the view point of photo energy transformation.Further, a photocatalyst which shows activity to the photo decomposingreaction of water can be recognized as an excellent photo functionalmaterial which provides function such as photo absorption, electrolyticseparation or oxidation-reduction reaction at the surface.

[0004] Kudo, Kato et al, are explaining that alkaline tantalate oralkaline earth element is a photocatalyst which shows excellent activityto the stoichiometric photocatalytic decomposing reaction of water byquoting various prior arts (for example, Catal. Lett., 58(1999),153-155; Chem. Lett., (1999), 1207; Surface, Vol.36, No.12 (1998),625-645 (hereinafter shortened to Document A)) . In above mentionedDocument A, there is an explanation about an useful photocatalyticmaterials for proceeding the decomposing reaction of water to hydrogenand/or oxygen using a photocatalyst, and many indications aboutphotocatalyst used for stoichiometric photocatalytic decomposingreaction of water are mentioned.

[0005] Further, a photocatalyst which loads a promoter such as platinumor NiO is referred.

[0006] However, the photocatalysts explained in these documents aremainly the compound containing oxygen as a non-metallic element and atransition metal such as Ta, Nb and Ti as a metal. And in the cases ofvarious solid photocatalyst, since the width of a forbidden band existsbetween a valence electron band and a conduction band, that is, band gapenergy is larger than 3eV, it is difficult to excite it by low energyunder 3eV, namely it is difficult to generate electrons and positiveholes by light. On the contrary, almost all of the conventional solidphotocatalyst which can generate electrons and holes by visible-lightirradiation are unstable under the condition of photocatalytic waterdecomposing reaction. For example, the band gap energy of CdS or Cu-ZnSis 2.4eV, but the catalytic reaction is restricted because it isaffected by photo-corrosive action, which is corrosive oxidative action.In the meanwhile, almost all of the solar light which reaches to thesurface of the earth is the visible light radiation of lower energy.Therefore, for the purpose to progress various photocatalytic reactionseffectively, a stable photocatalyst which acts by visible light isneeded. However, among the conventional technique, there is no techniqueto satisfy the above mentioned requirement.

[0007] The U.V.(ultra violet)-visible light absorption spectrum ofcadmium sulfide CdS on the market (99.99% purity) is already measuredand it is known that it absorbs the light of spectrum region from ultraviolet to visible light of 550nm. Further, according to the reports ofmany researchers, in the case of CdS, since a valence electron band anda conduction band, which forms band gap 2.4eV, has surplus to generateoxygen and hydrogen by electric potential view, theoretically it isdeemed to have an ability to decompose water to hydrogen and oxygen.However, there is a report which reports that when decomposing reactionof water is carried out by irradiating visible light over than 440nm,hydrogen can be obtained stable, but the generation of oxygen can not beobserved at all. This phenomenon can be explained as follows. That is,photo dissolution of catalyst itself, in other word, photo-corrosiveaction is caused because of instability of chemical species on catalystsurface and when a positive hole in the inside of solid generated byphoto excitation is transferred to the surface, it oxidizes a S²- on thesurface prior to a water molecule and forms a surface film.

[0008] From the above mentioned fact, it is obviously known that thepure sulfide CdS can reduce protons to hydrogen by visible light havinglonger wavelength than 440nm, but is not so stable material to generateoxygen from water and does not have such an ability.

[0009] The inventors of the present invention have conjectured thatsince a valence electron of a nitrogen atom has higher energy than thatof oxygen atom, a band gap energy of metal compound containing nitrogenatom can be make smaller than that of metal oxide, and considered that ametal and a metal compound bonded with adequate amount of nitrogen atomsbecome possible to generate excitation electrons and holes and willbecome a photocatalyst which acts by visible light irradiation. And theinventor of the present invention synthesized oxynitride containing atransition metal and proposed a photocatalyst which acts by visiblelight (refer to JP Application 2000-256681).

[0010] The object of the present invention is to provide a novelphotocatalyst which acts at broad wave length region of visible light.The inventors of the present invention have further proceeded an idea ofabove mentioned oxynitride containing transition metal, and think ofthat a photocatalyst which acts by visible light can be obtained from acompound wherein transition metal is bonded with nitrogen alone.Therefore, the inventors of the present invention have concentrated inthe investigation for the synthesis of stable transition metal nitride,and have found out that the compound containing rhombic tantalum nitrideof characterizable by chemical formula Ta₃N₅ can be used in stable as aphotocatalyst. Thus the object of the present invention is accomplished.The compound containing rhombic tantalum nitride means that the compoundis not necessary to be a pure rhombic tantalum nitride but is sufficientif the diffraction spectrum of tantalum nitride is detected by an X-raydiffraction.

DISCLOSURE OF THE INVENTION

[0011] The first one of the present invention relates to a photocatalystwhich contains at least rhombic tantalum nitride. Desirably, the firstone of the present invention is the photocatalyst containing at leastrhombic tantalum nitride which loads a promoter made of transitionmetal, and more desirably, the promoter being loaded on saidphotocatalyst containing at least rhombic tantalum nitride is platinum.

[0012] The second one of the present invention is a photocatalystcomprising rhombic tantalum nitride. Desirably, the second one of thepresent invention is the photocatalyst comprising rhombic tantalumnitride which loads a promoter made of transition metal, and moredesirably, the promoter being loaded on said photocatalyst comprisingrhombic tantalum nitride is platinum.

[0013] The third one of the present invention is a photocatalyst fordecomposition of water comprising the photocatalyst of the first andsecond inventions.

BRIEF ILLUSTRATION OF THE DRAWINGS

[0014]FIG. 1 is a graph showing the X-ray diffraction pattern of visiblelight photocatalyst Ta₃N₅ of the present invention.

[0015] FIG.2 is a graph showing the U.V. visible light diffusereflectance spectrum of Ta₃N₅ of the present invention.

[0016] FIG.3 is a graph showing hydrogen evolution rate by irradiatingthe visible-light longer than 420nm using 1 wt% Pt promoter loaded Ta₃N₅as a photocatalyst to 0.200dm³ of aqueous solution of 10 vol% methanolin which 0.2g of said catalyst is suspended.

[0017] FIG.4 is a graph showing oxygen evolution rate by irradiating thevisible-light longer than 420nm using 1 wt% Pt promoter loaded Ta₃N₅ asa photocatalyst to 0.200dm³ of aqueous solution of 0.01mol/dm³ AgNO₃ inwhich 0.2g of said catalyst is suspended.

[0018] FIG.5 is a graph showing the X-ray diffraction pattern of Ta₂ 0₅, which is the starting material of photocatalyst of the presentinvention.

[0019] FIG.6 is a graph showing the U.V. visible light diffusereflectance spectrum of photocatalyst Ta₂ 0 ₅, which is the startingmaterial of photocatalyst of the present invention.

[0020] FIG.7 is a graph showing the X-ray diffraction pattern of themixed powder of Ta₂ 0 ₅ and La₂O₃ which is calcined under ammonia gasflow for 2.5-20 hours.

[0021] FIG.8 is a graph showing hydrogen and oxygen evolution rate byirradiating the visible light longer than 420nm using 1 wt% Pt promoterloaded calcined mixed powder of FIG.7 as a visible light photocatalystto 0.200dm³ of aqueous solution of 10 vol% methanol in which 0.2g ofsaid catalyst is suspended and to 0.200dm³ of aqueous solution of0.01mol/dm³ AgNO₃ in which 0.2g of said catalyst is suspended.

THE BEST EMBODIMENT TO CARRY OUT THE INVENTION

[0022] The present invention will be illustrated more in details.

[0023] A. Tantalum nitride Ta₃N₅ which has photocatalytic function atvisible light of the present invention can be synthesized by thereaction of tantalum compound and nitrogen containing compound. As thestarting material of said tantalum compound, tantalum oxide, tantalumhalide, tantalate or tantalum complex are used. Tantalum nitride whichhas photocatalytic function at visible light of the present inventioncan be synthesized by reacting above mentioned starting material fortantalum compound or the mixture of said starting material for tantalumcompound with other metal oxide, metal halide or metallic salt withammonia, metallic amide or complex of metallic ammine.

[0024] Especially, the reaction of tantalum oxide with ammonia isadvantageous for the synthesis method of photocatalyst of the presentinvention. In this reaction, ammonia acts as a reducing agent and alsoas a nitriding agent.

[0025] The flow rate of ammonia depends on the reacting temperature.That is, when the temperature is high, the supplying speed becomes fast.The reacting temperature is within the limit of 400° C.-1200° C.(673-1473K).

[0026] B. The tantalum nitride obtained by above mentioned calcinating(nitriding) process, the catalyst activity of it can be improve byadding a promoter.

[0027] As the promoter, transition metal or transition metal compound,for example, platinum (Pt), nickel oxide (NiO) can be mentioned. Theadding amount of the promoter to the catalyst is within the limit from0.1wt% to 10wt%.

[0028] As the method for adding of the promoter, a conventional addingmethod of the promoter to the photocatalyst can be used. For example, inthe case of Pt, aqueous solution of tetraamminedichloroplatinum[(Pt(NH₃)₄Cl₂] is penetrated into catalyst and dried, then reduced byhydrogen, thus Pt can be added.

EXAMPLES

[0029] The present invention will be illustrated more in detailsaccording to the Examples. However, following Examples are mentioned forthe purpose to clearly explain the usefulness of the present inventionand not to intending to limit the scope of the present invention.

[0030] Example 1

[0031] Tantalum oxide Ta₂O₅ (1.00g) is heated to 850° C.. (1123K) by 10°C.. (10K)/minute temperature elevating rate and maintain thistemperature for 25hours under the ammonia NH₃ flow rate of 1dm³/min.,then quenched to the room temperature under He gas flow. Thus, Ta₃N₅material is synthesized. Platinum, which is a promoter, is added byfollowing process. That is, aqueous solution oftetraamminedichloroplatinum Pt(NH₃)₄Cl₂ is impregnated into obtainedTa₃N₅ material on a water bath and water is evaporated. Then reduced byhydrogen gas at 300° C. (573K) for 2 hours. The impregnating amount ofthe promoter can be altered within the limit of 0.110 wt%.

[0032] The X-ray diffraction pattern of calcined Ta₃N₅ material is shownin FIG. 1. All the diffraction peaks in FIG. 1 are assigned to those ofTa₃N₅ and the generation of rhombic Ta₃N₅ is recognized. That is, it isrecognized that the obtained product is composed of Ta₃N₅ on which Ptpromoter is loaded. U.V. -visible light diffuse reflectance spectrum ofsaid material is shown in FIG.2. It is obvious from FIG.2 that the abovematerial absorbs the visible light of shorter wavelength than 620nm. InFIG.3, hydrogen evolution rate by irradiating the visible light longerthan 420nm using 1 wt% Pt promoter loaded Ta₃N₅ as a visible lightphotocatalyst to 0.200dm³ of aqueous solution of 10vol.% methanol inwhich 0.2g of said catalyst is suspended is shown. As the light source,500W xenon lamp is used, and visible light of longer wavelength than420nm is irradiated by using a wavelength filter which cut off the lightof shorter wavelength than 420nm. As shown in FIG.3, it become clearthat above mentioned catalyst can generate hydrogen constantly fromaqueous solution of methanol under the irradiation of visible light oflonger wavelength than 420nm. In FIG.4, oxygen evolution rate byirradiating the visible light of longer wavelength than 420nm using 1wt% Pt promoter loaded Ta₃N₅ as a visible light photocatalyst to0.200dm³ of aqueous solution of 0.01mol/dm³ AgNO₃ in which 0.2g of saidcatalyst is suspended is shown. The reaction is carried out by the samecondition to above mentioned hydrogen generating experiment. As shown inFIG.4, it becomes clear that above mentioned catalyst can generateoxygen from aqueous solution of silver nitrate. As mentioned above, itis recognized that Ta₃N₅ has an ability to reduce proton to hydrogen andto oxidize water to oxygen by the visible light of longer wavelengththan 420nm. Further, in the case of irradiation of visible light oflonger wavelength than 600nm by passing through a wavelength filterwhich cuts off the visible light of shorter wavelength than 600nm, thesame results to the above mentioned case is obtained. In the meanwhile,in the case when visible light of longer wavelength than 700nm isirradiated, by passing through a wavelength filter which cuts off thevisible light of shorter wavelength than 700nm, the generation ofhydrogen or oxygen is not observed. Thus, calcined mixture powdercontaining Ta₃N₅ material is synthesized.

[0033] Example 2

[0034] The mixture of calcined mixture powder containing Ta₂O₅ (1.00g)powder and lanthanum oxide La₂O₃ (0.74g) powder is heated to 850° C.(1123K) by 10K/minute temperature elevating rate and maintain thistemperature for 2.5-20hours under the ammonia NH₃ gas flow rate of1dm³/min., then quenched to the room temperature under He gas flow. Pt,which is a promoter, is added according to following process. That is,aqueous solution of tetraamminedichloroplatinum Pt(NH₃)₄Cl₂ isimpregnated into obtained Ta₃N₅ material on a water bath and water isevaporated. Then reduced by hydrogen gas at 300° C. (573K) for 2 hours.The impregnating amount of the promoter can be altered within the limitof 0.110 wt.%.

[0035] X-ray diffraction patterns of the oxide powder mixture aftercalcined in ammonia for 2.5, 5, 10, 15 and 20 hours are shown in FIG.7.The diffraction peaks in FIG.7 are assigned to those of Ta₃N₅ (x), La₂O₃(O) and La(OH)₃, and the generation of rhombic Ta₃N₅ is recognized. Thatis, it is recognized that the obtained product is a product containingat least rhombic Ta₃N₅. The U.V..visible light diffuse reflectancespectrum of said material is measured, and the results indicate thatsaid material absorbs the visible light of shorter wavelength light than620nm likely to Example 1. In FIG.8, hydrogen and oxygen evolution rateobtained in each nitriding calcinating time of above mentioned material.The generation of hydrogen is carried out by suspending 0.2g ofphotocatalyst prepared by loading 1wt.% of Pt promoter on the materialcontaining Ta₃N₅ obtained by above mentioned nitriding and calcinatingprocess and by irradiating the visible light. In the meanwhile, thegeneration of oxygen is carried out by suspending 0.2g of abovementioned photocatalyst into 0.200dm³ of aqueous solution of 0.01NAgNO₃, and irradiating visible light. As the light source, 500W xenonlamp is used, and the reactor was irradiated with the visible light(wavelength longer than 420nm), using a wavelength cut-off filter. Asshown in FIG.8, it is understood that the above catalyst containingTa,N₅can generate hydrogen and oxygen by irradiation of visible light oflonger wavelength than 420nm. Further, from FIG.8, it is understood thatabove mentioned mixture indicates strong activity to the photooxidization of water by nitriding calcinating from 5 hours to 20 hoursunder the ammonia gas flow. As mentioned above, the metal oxidecontaining Ta₃N₅ is recognized to have an ability to reduce a proton tohydrogen and oxidize water to oxygen by visible light having longerwavelength than 420nm. Still more, by passing through a wavelengthfilter which cuts off the light of shorter wavelength than 600nm, thesame results to the above mentioned case is obtained in the case ofirradiation of visible light of longer wave length of longer than 600nm.Furthermore, in the case when visible light of longer wavelength than700nm is irradiated, even if the light is passed through a wavelengthfilter which cuts off the visible light of shorter wavelength than700nm, the generation of hydrogen or oxygen is not observed.

[0036] Control Example 1

[0037] In this Control Example, tantalum oxide on the market is used.Platinum, which is the promoter, is added by impregnatingtetraamminedichloroplatinum Pt(NH₃)₄Cl₂ on a water bath and byevaporating water. The obtained product is reduced by hydrogen for 2hours at 300° C. (573K). The impregnating amount of the promoter can bealtered in the limit of 0.1-5 wt.%.

[0038] The X-ray diffraction of calcined material is shown in FIG.5. Alldiffraction peaks in FIG.5 are almost belonging to Ta₂O₅ and thegeneration of Ta₂O₅is recognized. The U.V..visible light diffusereflectance spectrum of said material is shown in FIG.6. From FIG.6, itis understood that the above material absorbs the U.V. light of shorterwavelength than 270nm. When the reaction is carried out by irradiationof visible light like to the Examples, the generation of H₂ and O₂ arenot observed.

[0039] From the above mentioned results, Ta₃N₅ prepared by replacing alloxygen of Ta₂O₅ to nitrogen is recognized to have an ability to reduce aproton to hydrogen and oxidize water to oxygen by visible light havinglonger wavelength than 420nm.

INDUSTRIAL APPLICABILITY

[0040] As mentioned above, the photocatalyst obtained by the presentinvention, is the catalyst that acts by visible light, which is themajority in sun light, reaching to the surface of the earth. By carryingout photocatalyst reaction with sun light, the useful compound can beproduced. Further, as indicated in Examples, since said photocatalysthas an ability to decompose water to hydrogen and oxygen by visibleradiation, it is hopeful to be used as a photocatalyst convert sun lightto hydrogen which is considered as the energy of next generation. Inthis connection, the ratio of U.V. light to the whole sun light isapproximately 5%, and it is obvious that the use of visible light isremarkably effective.

What is claim
 1. A photocatalyst which contains at least rhombictantalum nitride.
 2. The photocatalyst containing at least rhombictantalum nitride of claim 1, wherein said photocatalyst loads a promotercomposed of transition metal.
 3. The photocatalyst for decomposition ofwater comprising the photocatalyst of claim
 2. 4. The photocatalystcontaining at least rhombic tantalum nitride of claim 2, wherein thepromoter is platinum.
 5. The photocatalyst for decomposition of watercomprising the photocatalyst of claim
 4. 6. A photocatalyst comprisingrhombic tantalum nitride.
 7. The photocatalyst comprising rhombictantalum nitride of claim 6, wherein said photocatalyst loads a promotercomposed of transition metal.
 8. The photocatalyst comprising rhombictantalum nitride of claim 7, wherein the promoter is platinum.
 9. Thephotocatalyst for decomposition of water comprising the photocatalyst ofclaim 8.